Mucosal HIV exposures are responsible for the vast majority of HIV-1 infections worldwide. Specifically, vaginal intercourse is the most common form of HIV transmission. Therefore, prevention of sexual transmission is urgently needed. Despite the highly encouraging results from the CAPRISA 004 trial it is clear that the level of protection observed is suboptimal and that there is significant room for further improvement. In addition, Tenofovir, the chemo-prophylactive agent being used (alone or in combination) in ongoing randomized clinical trials of pre-exposure prophylaxis (PrEP) is also being widely used as front line therapy for HIV infection. In this proposal we combine the strengths of four outstanding investigators at Merck and UNC with highly complementary skills to address, using highly innovative approaches, issues of fundamental importance to the development of the next generation of PrEP agents. Specifically, 1) a novel second generation integrase inhibitor (MK-2048) developed by Daria Hazuda at Merck that has not been used for treatment or for PrEP and that has a resistance profile that is different from other integrase inhibitors; 2) at UNC, a highly innovative in vivo model of mucosal HIV transmission developed by J. Victor Garcia that will for the first time utilize cell free and cell associated primary transmitted/founder (F/T) viruses for challenge in the presence of semen; 3) state-of-the-art analytical methods developed by Angela Kashuba at UNC to determine the PD/PK of vaginally applied MK-2048 to establish correlates of in vivo protection; and 4) mucoadhesive gel formulations and intravaginal rings developed by Russ Mumper at UNC for the optimized release of MK-2048 capable of penetrating the tissues of the female reproductive tract to establish an effective pharmacological barrier. In order to attain these goals we propose the following specific aims: Specific Aim 1) To develop the basic and preclinical framework for the development and in vivo testing of new generations of PrEP agents. Specific Aim 2) To develop aqueous-based mucoadhesive gel formulations of the integrase inhibitor MK- 2048 for vaginal administration. Specific Aim 3) To conduct proof-of-concept safety and efficacy studies of MK-2048 in humanized BLT mice. Specific Aim 4) To generate pharmacokinetic-pharmacodynamic (PK-PD) models to assess the protective effect of MK-2048 in the female reproductive tract of humanized BLT mice.